1. Field of the Invention
The present invention relates generally to an apparatus for controlling a rotation speed of a recording medium drive motor in an information signal reproducing apparatus.
2. Description of the Prior Art
A disc is known on which an audio signal and other information signal are digitized and recorded. An optical disc for the audio disc, for example, is known on which the audio signal is pulse code modulated and then recorded. To record a pulse code modulation (hereinafter simply refered to PCM) signal on the disc, a method of recording at a constant angular velocity (hereinafter simply refered to CAV) and a method of recording at a constant linear velocity (hereinafter simply refered to CLV) are proposed.
In the CAV system, a rotation speed of the disc drive motor is maintained constant over an entire reproduction range. While in the CLV system, it is necessary to decrease a rotation speed of the motor as a pick-up goes toward the outer periphery so that the relative speed between the disc and the pick-up, that is, a linear speed is kept constant and the clock rate of the reproduced signal is kept constant. In the CAV system, an information density in a unit rack length is lower in an outer periphery than in an inner periphery, while in the CLV system it is constant over entire range. Accordingly, the recording at CLV is preferable.
The disc on which the PCM signal is recorded at the CLV has to be reproduced at the CLV. Such a method for controlling the disc upon playback is known that a position of a pick-up is detected by a potentiometer and since the necessary rotational number becomes the inverse number of such position, the detected output is supplied to a divider so as to obtain a control information. This method, however, needs such an arrangement consisting of the position detector or potentiometer and the divider, so this leads to expensive and complex arrangement.
Therefore, it is proposed to use the reproduced signal from the disc so as to control the disc rotation at the CLV without using the detector for detecting the position of the pick-up, in order to overcome the drawbacks as mentioned above. That is, a method for obtaining a control information or data based upon the reproduced signal from the disc has been proposed. For instance, in a digital audio disc (hereinafter simply refered as DAD) on an optical signal detection system, a sync signal is accompanied in every frame, for respective digitized audio signals, i.e., the PCM signals, with a predetermined frequency. In this case, the PCM signal is recorded at the base band system which is not a carrier modulation such as amplitude and frequency modulations. The modulation method of the case is used normally such as a run length limited code.
The run length limited code modulation method is, with respect to data "1" or "0", a minimum transition interval T.sub.min between two data is extended to enhance the recording efficiency and a maximum transition interval T.sub.max therebetween is shortened to facilitate a self clock on a playback side. The PCM signal varies at its pulse waveform between T.sub.min and T.sub.max, however the duty ratio of the PCM signal is kept in 50% within a predetermined successive number of the frames. While the sync signal is always formed in a waveform consisting of a pair of positive and a negative pulses of equal length, that is, 50% duty ratio in its positive and negative periods.
Then, a conventional motor speed control method has used the sync signal as the control information. However, the method has not been sufficient for the control of the motor at the CLV. Because the sync signal is not obtainable at cases such a time of starting the motor and a time of forwarding fast the pick-up. The conventional method has also same problem for signal dropouts caused, for example, by dusts or scratches on the disc.
To comply with the problem of the conventional method as mentioned above, we have developed another method for control of the motor at the CLV. The method is described in Japanese Patent Application Laid-Open No. 58-220271. In the method, T.sub.max of the PCM audio signal at its positive interval or negative interval is detected and then the motor speed is controlled so as the T.sub.max to be kept in a predetermined value. However, we have found a problem still left in the method. That is, the PCM audio signal is obtained by slicing a reproduced signal from the disc at a predetermined reference slice level. When the reproduced signal varies at its level, the positive or negative interval of the PCM signal varies in accordance with a relative change between the reproduced signal and the reference slice level.
Therefore, T.sub.max detected based on the interval also varies so that the motor speed is wrongly changed.